Cloned cDNAs have proved to be an extremely powerful tool to search for, isolate and analyze mammalian genes. The presently used methods for cloning cDNAs, however, are limited by the ability to detect particular cDNA clones; currently, nucleic acid hybridization and immuno-detection of products of in vitro translation or expression in E. coli are the sole means to identify clones. In order to mitigate this limitation we are attempting to develop an alternative approach that permits cloning cDNAs based on their functional expression in mammalian cells, taking advantage of the fact that cDNAs have the ability to encode functional proteins. To pursue this approach three key techniques need to be developed: 1) a method that permits high efficiency cloning of full length cDNAs; 2) a cDNA cloning vector that permits expression of cDNA inserts in mammalian cells; 3) a method and/or vector for efficient transfection of mammalian cells with a cDNA library constructed in an expression vector. As to points 1) and 2), we have already completed most if not all of the required experiments. We are currently constructing a lambda phage vector for efficient tranfection and easy rescue of integrated cDNA genes. The lambda vector contains a bacterial neomycin resistance gene under the control of the SV40 early promoter as a dominant selectable marker gene, and a unique Sal I restriction site that can be used to insert the entire cDNA-expression vector recombinant molecule. The test of the function of the vector is in progress using human HGPRT cDNA cloned in the pcD expression vector as a model molecule.